In recent years, electronic devices are highly required to be increased in integration in the electronic device production field represented by integrated circuit device production, and this allows a photolithographic technique for forming fine patterns to be required. Accordingly, photoresist compositions corresponding to photolithography using as exposure light, radial rays having a wavelength of 200 nm or less such as an ArF excimer laser (wavelength: 193 nm), an F2 excimer laser (wavelength: 157 nm) and the like are actively developed, and proposed are a large number of chemically amplified photoresist compositions comprising polymer having an acid-dissociable functional group and compounds (herein referred to as “a photoacid generator”) generating acid by irradiation (herein referred to as “exposure”) of a radial ray. The above polymer having an acid-dissociable functional group comprises a basic structure in which a part of an alkali-readily soluble site of an alkali-soluble polymer is protected by a suitable acid-dissociable functional group, and selection of the above acid-dissociable functional group is very important in terms of controlling the performances of the photoresist composition.
Known as the existing acid-dissociable functional group are 1) groups having an adamantane structure (refer to a patent document 1 and a non-patent document 1) and 2) groups comprising a tetrahydropyranyl group (refer to a patent document 2). The acid-dissociable functional group is required to allow a high reactivity to acids to be consistent with a stability in which it is not decomposed at a baking step and requested to have a heat stability of 130° C. or higher (refer to a non-patent document 3). The tetrahydropyranyl group in 2) has the advantage that it has a high reactivity in terms of an acid dissociation, but it is lacking in a heat stability and is not satisfactory in a fundamental performance of the resist.
One of large problems of lithographic techniques in recent years includes line width variation of formed patterns which is called a line width roughness (herein referred to as “LWR”), and an allowable value thereof is required to be less than 8% of a line width (refer to a non-patent document 3). It is necessary for improving LWR to inhibit patterns—from being deformed by swelling, that is, to allow a polymer which is a photoresist composition component to be less liable to be swollen.
A polymer into which 1) the group having an adamantane structure is introduced as the acid-dissociable functional group has a high reactivity to acids and a heat stability. However, the above polymer has a high hydrophobicity and is not satisfactory in an affinity with a developer to allow parts which are not dissolved in developing to remain in an exposed area, and it brings about swelling to result in causing a problem of increasing LWR. Accordingly, polymers for a photoresist composition which are less liable to be swollen are still anxious to be developed, and the existing situation is that compounds having an acid-dissociable functional group for achieving the above matter are strongly anxious to be developed.
Further, finer resist patterns (for example, fine resist patterns having a line width of about 90 nm) shall be required to be formed in the future. In order to achieve formation of resist patterns having a finer line width than 90 nm, it is considered to shift a wavelength of a light source in an exposure equipment to a shorter region and increase a numerical aperture (NA) of a lens. However, a new expensive exposure equipment is required for shifting a wavelength of a light source to a shorter region. Further, in an increase in a numerical aperture of a lens, a resolution and a depth of focus in a relation of trade-off, and therefore the problem that the depth of focus is reduced even if the resolution is elevated is involved therein.
In recent years, a method called a liquid immersion lithography is reported as a lithographic technique which makes it possible to solve the above problem. This method is a method in which purified water or a liquid refractive medium (immersion liquid) such as a fluorinated inert liquid having a prescribed thickness is allowed to be present at least on a photoresist film between a lens and a photoresist film on a substrate in exposure. In the above method, even if a light source having the same exposing wavelength is used, a higher resolving property is achieved (provided with a high resolution) as well as having no change in a depth of focus as is the case with an instance in which a light source having a shorter wavelength is used and an instance in which a high NA lens is used by substituting an space of exposure optical path which has so far been an inert gas such as air and nitrogen with a liquid having a larger refractive index (n), for example, purified water and the like. Use of the above liquid immersion lithography makes it possible to achieve formation of a resist pattern which is formed at a lower cost and is excellent in a higher resolving property and which is excellent as well in a depth of focus by using a lens mounted in an existing equipment, and therefore it attracts attentions very much.
On the other hand, if a refractive index of a liquid refractive index medium (immersion liquid) is higher than a refractive index of, for example, a photoresist film in a liquid immersion lithographic process, light is less liable to be incident from an immersion liquid into the photoresist film according to a Snell's law. Accordingly, the fundamental performances such as the sensitivity and the like are likely to be deteriorated. Further, if an immersion liquid has a high refractive index, a difference in a refractive index between the immersion liquid and the photoresist film is increased, and light is reflected wholly on an interface between the immersion liquid and the photoresist film. Accordingly, since light is not incident completely into the photoresist film, the sufficiently high sensitivity is not obtained, and it is anticipated that a throughput in a resist process is notably reduced.
Then, it is proposed that particularly when an immersion liquid (immersion liquid having a high refractive index) having a refractive index of 1.70 or more in a wavelength of 193 nm is used, a photoresist film having a higher refractive index than that of the above immersion liquid is used (refer to non-patent documents 4 and 5).    Patent document 1: Japanese Patent Application Laid-Open No. 73173/1997    Patent document 2: Japanese Patent Application Laid-Open No. 88367/1993    Non-patent document 1: Journal of Photopolymer Science and Technology, Vol. 9, No. 3, p. 475 to 487 (1996)    Non-patent document 2: ITRS 2006, UP DATE version, part of lithography, p. 8    Non-patent document 3: ITRS 2006, UP DATE version, part of lithography, p. 7    Non-patent document 4: SPIE 2006 61530H    Non-patent document 5: SPIE 2006 61531L